Carbon anode compositions for lithium ion batteries

ABSTRACT

A lithium secondary battery comprising a positive electrode, a negative electrode comprising a carbonaceous material which is capable of absorbing and desorbing lithium ions, and a non-aqueous electrolyte disposed between the negative electrode and the positive electrode. The carbonaceous material comprises a graphite crystal structure having an interplanar spacing d 002  of at least 0.400 nm (preferably at least 0.55 nm) as determined from a (002) reflection peak in powder X-ray diffraction. This larger interplanar spacing implies a larger interstitial space between two graphene planes to accommodate a greater amount of lithium. The battery exhibits an exceptional specific capacity, excellent reversible capacity, and long cycle life.

This invention is based on the research result of a US FederalGovernment Small Business Innovation Research (SBIR) project. The USgovernment has certain rights on this invention.

FIELD OF THE INVENTION

The present invention provides a carbonaceous material for use as ananode active material in a secondary battery, particularly lithium-ionbattery.

BACKGROUND

The description of prior art will be primarily based on the list ofreferences presented at the end of this section.

Concerns over the safety of earlier lithium secondary batteries led tothe development of lithium ion secondary batteries, in which purelithium metal sheet or film was replaced by carbonaceous materials asthe anode. The carbonaceous material may comprise primarily graphitethat can be intercalated with lithium and the resulting graphiteintercalation compound may be expressed as Li_(x)C₆, where x istypically less than 1. In order to minimize the loss in energy densitydue to this replacement, x in Li_(x)C₆ must be maximized and theirreversible capacity loss Q_(ir) in the first charge of the batterymust be minimized. Carbon anodes can have a long cycle life due to thepresence of a protective surface-electrolyte interface layer (SEI),which results from the reaction between lithium and the electrolyteduring the first several cycles of charge-discharge. The lithium in thisreaction comes from some of the lithium ions originally intended for thecharge transfer purpose. As the SEI is formed, the lithium ions becomepart of the inert SEI layer and become irreversible, i.e, they can nolonger be the active element for charge transfer. Therefore, it isdesirable to use a minimum amount of lithium for the formation of aneffective SEI layer. In addition to SEI formation, Q_(ir) has beenattributed to graphite exfoliation caused by electrolyte solventco-intercalation and other side reactions [Refs. 1-4].

The maximum amount of lithium that can be reversibly intercalated intothe interstices between graphene planes of a perfect graphite crystal isgenerally believed to occur in a graphite intercalation compoundrepresented by Li_(x)C₆ (x=1), corresponding to a theoretical specificcapacity of 372 mAh/g. In other graphitized carbon materials than puregraphite crystals, there exists a certain amount of graphitecrystallites dispersed in or bonded by an amorphous or disordered carbonmatrix phase. The amorphous phase typically can store lithium to aspecific capacity level higher than 372 mAh/g, typically up to 700mAh/g, although a specific capacity higher than 1,000 mAh/g has beensporadically reported. Hence, the magnitude of x in a carbonaceousmaterial Li_(x)C₆ varies with the proportion of graphite crystallitesand can be manipulated by using different processing conditions, asexemplified in [Refs. 1-4]. An amorphous carbon phase alone tends toexhibit a low electrical conductivity (high charge transfer resistance)and, hence, a high polarization or internal power loss. Conventionalamorphous carbon-based anode materials also tend to give rise to a highirreversible capacity.

The so-called “amorphous carbons” commonly used as anode activematerials are typically not purely amorphous, but contain some micro- ornano-crystallites with each crystallite being composed of a small numberof graphene sheets (basal planes) that are stacked and bonded togetherby weak van der Waals forces. The number of graphene sheets variesbetween one and several hundreds, giving rise to a c-directionaldimension (thickness Lc) of typically 0.34 nm to 100 nm. The length orwidth (La) of these crystallites is typically between tens of nanometersto microns. Among this class of carbon materials, soft and hard carbonsmade by low-temperature pyrolysis (550-1,000° C.) exhibit a reversiblecapacity of 400-800 mAh/g in the 0-2.5 V range [Refs. 1-3]. Dahn et al.have made the so-called house-of-cards carbonaceous material withenhanced capacities approaching 700 mAh/g [Refs. 1, 2]. Tarascon'sresearch group obtained enhanced capacities of up to 700 mAh/g bymilling graphite, coke, or carbon fibers [Ref. 3]. Dahn et al. explainedthe origin of the extra capacity with the assumption that in disorderedcarbon containing some dispersed graphene sheets (referred to ashouse-of-cards materials), lithium ions are adsorbed on two sides of asingle graphene sheet [Refs. 1, 2]. It was also proposed that Li readilybonded to a proton-passivated carbon, resulting in a series ofedge-oriented Li—C—H bonds. This provides an additional source of Li⁺ insome disordered carbons [Ref. 5]. Other researchers suggested theformation of Li metal mono-layers on the outer graphene sheets [Ref. 6]of graphite nano-crystallites. The amorphous carbons of Dahn et al. wereprepared by pyrolyzing epoxy resins and may be more correctly referredto as polymeric carbons. Polymeric carbon-based anode materials werealso studied by Zhang, et al. [Ref. 16] and Liu, et al. [Ref. 17].

Peled and co-workers improved the reversible capacity of a graphiteelectrode to 400 mAh/g by mild air oxidation [Ref. 4]. They showed thatmild oxidation (burning) of graphite produces well-defined voids ornano-channels, having an opening of a few nanometers and up to tens ofnanometers, on the surface of the graphite. They believed that thesenano-channels were small enough to prevent co-intercalation of thesolvent molecule but large enough to allow Li-ion penetration [Ref. 4].These nano-channels were formed at the La-Lc interface, called “zigzagand armchair faces” between two adjacent crystallites, and in thevicinity of defects and impurities. Both natural and synthetic graphitematerials typically have a wide variety of functional groups (e.g.,carbonate, hydrogen, carboxyl, lactone, phenol, carbonyl, ether, pyrone,and chromene) at the edges of crystallites defined by La and Lc [Ref.7]. These groups can react with lithium and/or electrolyte species toform a so-called in situ CB-SEI (chemically bonded solid electrolyteinterface) [Ref. 4] on which, for example, carboxylic acid surface filmsare converted into Li-carboxylic salts.

Subsequently, several research groups have followed similar approachesto prepare mildly oxidized graphite anodes using a gaseous or liquidoxidant [Refs. 7-15]. For instance, Wu, et al. [Refs. 12-15] studied theelectrochemical behaviors of natural graphite treated with (NH₄)₂S₂O₈,Ce(SO₄)₂, and H₂O₂. They similarly concluded that mild oxidation servesto (1) remove some active sites or defects in graphitic materialsresulting in an improved surface structure; (2) form a dense layer ofoxides acting as an efficient passivating film; and (3) producenano-channels or micro-pores as storage sites and passages for lithium.

In summary, in addition to the above-cited three mechanisms, thefollowing mechanisms for the extra capacity over the theoretical valueof 372 mAh/g have been proposed [Ref. 4]: (i) lithium can occupy nearestneighbor sites; (ii) insertion of lithium species into nano-scaledcavities; (iii) in very disordered carbons containing large fractions ofsingle graphene sheets (like the structure of a house of cards) lithiummay be adsorbed on both sides of single layer sheets [Refs. 1, 2]; (iv)correlation of H/C ratio with excess capacity led to a proposal thatlithium may be bound somehow in the vicinity of the hydrogen atoms(possible formation of multi-layers of lithium on the external grapheneplanes of each crystallite in disordered carbons) [Ref. 6]; and (vi)accommodation of lithium in the zigzag and armchair sites [Ref. 4].

It is of significance to emphasize that the approach of mild oxidationcauses the formation of nano-channels at the zigzag and armchair facesbetween two adjacent crystallites and in the vicinity of defects andimpurities [e.g., as stated in the Abstract of Ref. 4]. X-raydiffraction studies indicate that “the chemical oxidation of thegraphite powder does not involve the formation of an intermediarygraphite intercalation compound (GIC), since the interlayer spacingremains constant (3.354 Å)” [Second paragraph of the right column, Page2969 of Ref. 7]. This implies that the bulk of the crystallites remainsintact; only the amorphous phase, defects and impurities, and theinterface between a crystallite and the amorphous phase have beenmodified. The reversible capacity of the resulting mildoxidation-treated graphite anode rarely exceeds 400 mAh/g [Refs. 4, 9]and, in most cases, still falls short of 372 mAh/g.

REFERENCES

-   1. T. Zheng, Q. Zhong, and J. R. Dahn, J. Electrochem. Soc.    142 (1995) L211.-   2. J. S. Xue and J. R. Dahn, J. Electrochem. Soc. 142 (1995) 3668.-   3. F. Disma, L. Aymard, and J.-M. Tarascon, J. Electrochem. Soc.,    143 (1996) 3959.-   4. E. Peled, C. Menachem, A. Melman, J. Electrochem. Soc. 143 (1996)    L4.-   5. U. Rothlisberger and M. L. Klein, J. Am. Chem. Soc., 117, 42    (1995).-   6. R. Yazami and M. Deschamps, J. Power Sources, 54 (1995) 411.-   7. Y. Ein-Eli, V. R. Koch, J. Electrochem. Soc. 144 (1997) 2968.-   8. C. Menachem, Y. Wang, J. Floners, E. Peled, S. G. Greenbaum, J.    Power Sources, 76 (1998) 180.-   9. H. Buqa, P. Golob, M. Winter, J. O. Bensenhard, J. Power Sources,    97-98 (2001) 122.-   10. T. Takamura, H. Awano, T. Ura, K. Sumiya, J. Power Sources,    68 (1997) 114.-   11. Y. P. Wu, C. Jiang, C. Wan and E. Tsuchida, “Effects of    catalytic oxidation on the electrochemical performance of common    natural graphite as an anode material for lithium ion batteries,”    Electrochem. Commu., 2 (2000) 272-275.-   12. Y. P. Wu, C. Jiang, C. Wan and E. Tsuchida, “A Green Method for    the Preparation of Anode Materials for Lithium Ion Batteries,” J.    Materials Chem., 11 (2001) 1233-1236.-   13. Y. P. Wu, C. Y. Jiang, C. R. Wan, R. Holze, J. Appl.    Electrochem., 32 (2002) 1011.-   14. Y. P. Wu, C. Jiang, C. Wan, and R. Holze, Electrochem. Commu.,    4 (2002) 483-487.-   15. Y. P. Wu, E. Rahm, and R. Holze, J. Power Source, 114 (2003)    228-236.-   16. Zhang, et al., “Carbon Electrode Materials for Lithium Battery    Cells and Method of Making Same,” U.S. Pat. No. 5,635,151 (Jun. 3,    1997).-   17. Lui, et al., “Composite Carbon Materials for Lithium Ion    Batteries, and Method of Producing Same,” U.S. Pat. No. 5,908,715    (Jun. 1, 1999).-   18. R. Tossici, et al., “Lithium-ion Rechargeable Battery with    Carbon-Based Anode,” U.S. Pat. No. 6,087,043 (Jul. 11, 2000).-   19. Tanaka, et al., “Carbon Anode for Secondary Battery,” U.S. Pat.    No. 5,344,726 (Sep. 6, 1994).-   20. Nishimura, et al., “Nonaqueous Secondary Battery and a Method of    Manufacturing a Negative Electrode Active Material,” U.S. Pat. No.    5,965,296 (Oct. 12, 1999).-   21. Yamada, et al., “Nonaqueous Secondary Battery,” U.S. Pat. No.    6,040,092 (Mar. 21, 2000).-   22. Kawakubo, et al., “Cathode Formed of Graphite/Carbon Composite    for Lithium Ion Secondary Battery,” U.S. Pat. No. 6,139,989 (Oct.    31, 2000).-   23. Touzain, et al., “Insertion Compounds of Graphite with Improved    Performances and Electrochemical Applications of those Compounds,”    U.S. Pat. No. 4,584,252 (Apr. 22, 1986).-   24. N. Watanabe, et al., “High Energy Density Battery,” U.S. Pat.    No. 3,700,502 (Oct. 24, 1972).-   25. Watanabe, et al., “Poly-Dicarbon Monofluoride,” U.S. Pat. No.    4,139,474 (Feb. 13, 1979).-   26. Watanabe, et al., “Poly-Dicarbon Monofluoride,” U.S. Pat. No.    R30,667 (Jul. 7, 1981).-   27. Watanabe, et al., “Electrolytic Cell of High Voltage,” U.S. Pat.    No. 4,247,608 (Jan. 27, 1981).-   28. Watanabe, et al., “Process for Preparing Poly-Dicarbon    Monofluoride,” U.S. Pat. No. 4,243,615 (Jan. 6, 1981).-   29. Watanabe, et al., “Process for Producing a Graphite Fluoride    Comprising Mainly Polydicarbon Monofluoride Represented by the    Formula (C2F)n,” U.S. Pat. No. 4,423,261 (Dec. 27, 1983).-   30. Watanabe, et al., “Method for Producing Graphite Fluoride,” U.S.    Pat. No. 4,753,786 (Jun. 28, 1988).

SUMMARY OF THE INVENTION

The present invention provides a negative electrode (anode) materialcomposition for use in a lithium secondary battery. The compositioncomprises a carbonaceous material that is capable of absorbing anddesorbing lithium ions. This carbonaceous material comprises a graphitecrystal structure having an interplanar spacing d₀₀₂ of at least 0.400nm, as determined from a (002) reflection peak in powder X-raydiffraction. This interplanar spacing is also referred to as inter-layeror inter-graphene spacing, which is half of the height, C/2, of agraphite unit cell structure. This spacing is preferably at least 0.400nm, more preferably larger than 0.55 nm.

In one preferred embodiment, the carbonaceous material comprises amaterial derived from natural graphite, synthetic graphite, highlyoriented pyrolytic graphite, graphite fiber, carbon fiber, carbonnano-fiber, graphitic nano-fiber, spherical graphite or graphiteglobule, meso-phase micro-bead, meso-phase pitch, graphitic coke, orpolymeric carbon. For instance, natural flake graphite may be subjectedto a deep oxidation treatment under a condition comparable to what hasbeen commonly employed to prepare the so-called expandable graphite orstable graphite intercalation compound (GIC), but preferably with ahigher degree of oxidation, to obtain a true graphite oxide essentiallythrough out the bulk of the material. This can be accomplished, forinstance, by immersing graphite powder in a solution of sulfuric acid,nitric acid, and potassium permanganate for preferably 2-24 hours(details to be described later). The resulting acid-intercalatedgraphite compound is then subjected to vigorous washing and rinsing toremove essentially all the intercalants. The subsequently dried productis a heavily oxidized graphite powder, hereinafter referred to asgraphite oxide (GO), which comprises carbon and oxygen (at a C/O weightratio of typically 2.0/1.0 to 2.9/1.0) that is essentially sulfur-freeand nitrogen-free. Powder X-ray diffraction indicates that theinterplanar spacing is typically between 0.55 and 0.75 nm. Theseexpanded interplanar spacings provide interstitial spaces that are ideallocations to accommodate lithium ions, atoms, or molecules. They appearto be large enough to accommodate at least two “layers” of lithium, asopposed to just one layer of sporadically distributed lithium atoms in aconventional interstitial space (of approximately 0.27 nm as part of aninterplanar spacing of typically 0.335 nm) in either an untreatedgraphite or a so-called mildly oxidized graphite (wherein theinterplanar spacing remaining as approximately <0.34 nm) [Refs. 4,7-15].

Other examples of carbonaceous anode materials include graphite fluoride(GF), other halogenated graphite compounds, and other graphite compoundsthat have an interplanar spacing of 0.55 nm or above. Although graphiteoxide (the type that exhibits a narrow particle size range and highspecific surface area only [Ref. 23]) and certain types of graphitefluoride [Refs. 24-30] have been used as a positive electrode (cathode)active material in a lithium metal battery, these two groups ofmaterials have never been used as an anode (negative electrode) activematerial in rechargeable lithium-ion batteries. There has been noimplicit or explicit indication that either graphite fluoride orgraphite oxide could be a potential anode active material. It has notbeen trivial or obvious that a lithium-ion secondary battery featuringeither class of material would have a high specific capacity greaterthan the theoretical maximum of 372 mAh/g, have a good reversiblecapacity, and a stable cycling behavior. We have surprisingly discoveredthese features only after intensive research and development efforts.

The carbonaceous material may be derived from particles or flakes ofnatural graphite, synthetic graphite, highly oriented pyrolyticgraphite, graphite fiber, carbon fiber, carbon nano-fiber, graphiticnano-fiber, spherical graphite or graphite globule, meso-phasemicro-bead (MCMB), meso-phase pitch, graphitic coke, or polymericcarbon. Meso-phase pitch, graphitic coke, or polymeric carbon mayrequire additional graphitization treatment, typically at a temperaturein the range of 1,500 and 3,000° C. prior to a deep oxidation orfluorination treatment. After the presently invented treatment (deepoxidation, fluorination, halogenation, etc), the graphite crystals(micro- or nano-crystallites), dispersed in an amorphous matrix, willexhibit expanded interstitial spaces, also characterized by a typicalinterplanar spacing of 0.55-0.9 nm. Thus, preferably, the carbonaceousmaterial comprises a graphite oxide, graphite fluoride, orgraphite-halogen compound (or domains of graphite oxide, graphitefluoride, or stable graphite-halogen compound in an amorphous carbonmatrix).

A graphite material (e.g., natural graphite particle) is typicallycomposed of graphite crystals dispersed in or connected to an amorphousphase, along with other defects. Graphite particles (e.g., naturalgraphite flakes, MCMB particles, or artificially made graphite globules)may be mixed with a resin to form a composite. This composite may beheated at a temperature of typically 500-1,200° C. to convert the resininto a polymeric carbon or an amorphous carbon phase. Hence, in thepresently invented negative electrode material composition, thecarbonaceous material may further comprise an amorphous carbon phase orpolymeric carbon, wherein the graphite particle or a graphite crystalstructure therein is dispersed in or bonded by an amorphous carbon phaseor polymeric carbon.

Alternatively, the amorphous carbon phase may be obtained from chemicalvapor deposition, chemical vapor infiltration, or pyrolyzation of anorganic precursor. Further alternatively, the carbonaceous material maycomprise an electrically conductive binder material, wherein thegraphite crystal structure is dispersed in or bonded by this conductivebinder material. An electrically conductive binder material may beselected from coal tar pitch, petroleum pitch, meso-phase pitch, coke, apyrolyzed version of pitch or coke, or a conjugate chain polymer(intrinsically conductive polymer such as polythiophene, polypyrrole, orpolyaniline.

In the preparation of a negative electrode material, typically carbonparticles are bonded by a non-conductive material, such aspolyvinylidene fluoride (PVDF), to form an integral anode member. Hence,the carbonaceous material may further comprise a non-conductive bindermaterial, wherein the graphite crystal structure is dispersed in orbonded by the non-conductive binder material.

Another preferred embodiment of the present invention is a lithiumsecondary battery comprising a positive electrode, a negative electrodecomprising a carbonaceous material which is capable of absorbing anddesorbing lithium ions, and a non-aqueous electrolyte disposed betweenthe negative electrode and the positive electrode, wherein thecarbonaceous material comprises a graphite crystal structure having aninterplanar spacing d₀₀₂ of at least 0.400 nm (preferably at least 0.55nm) derived from a (002) reflection peak in powder X-ray diffraction.The negative electrode or anode material has the afore-describedcharacteristics. The carbonaceous material in the present inventionprovides a specific capacity of typically greater than 500 mAh/g, oftengreater than 650 mAh/g, and even greater than 800 mAh/g, which all farexceed the theoretical specific capacity of 372 mAh/g for graphite anodematerial. They also exhibit super multiple-cycle behaviors with a smallcapacity fade and a long cycle life.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Schematic of a cylinder-shape lithium ion battery.

FIG. 2 X-ray diffraction curves for natural graphite and graphite oxidesamples, indicating the shift of a (002) plane-induced peak from aninterplanar spacing of 0.335 nm for pristine graphite to 6.5-7.5 nm fordeeply oxidized graphite.

FIG. 3 The specific capacity of several groups of carbonaceous anodematerials plotted as a function of the interplanar spacing.

FIG. 4 The reversible specific capacity (after first cycle) of severalgroups of carbonaceous anode materials plotted as a function of theinterplanar spacing.

FIG. 5 The specific capacity of carbon anode materials based on naturalgraphite and its oxide version.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

This invention is related to a high-capacity lithium secondary battery,which is preferably a secondary battery based on a non-aqueouselectrolyte or a polymer gel electrolyte. The shape of a lithiumsecondary battery can be cylindrical, square, button-like, etc. Thepresent invention is not limited to any battery shape or configuration.

As an example, a cylindrical battery configuration is shown in FIG. 2. Acylindrical case 10 made of stainless steel has, at the bottom thereof,an insulating body 12. An assembly 14 of electrodes is housed in thecylindrical case 10 such that a strip-like laminate body, comprising apositive electrode 16, a separator 18, and a negative electrode 20stacked in this order, is spirally wound with a separator being disposedat the outermost side of the electrode assembly 14. The cylindrical case10 is filled with an electrolyte. A sheet of insulating paper 22 havingan opening at the center is disposed over the electrode assembly 14placed in the cylindrical case 10. An insulating seal plate 24 ismounted at the upper opening of the cylindrical case 10 and hermeticallyfixed to the cylindrical case 10 by caulking the upper opening portionof the case 10 inwardly. A positive electrode terminal 26 is fitted inthe central opening of the insulating seal plate 24. One end of apositive electrode lead 28 is connected to the positive electrode 16 andthe other end thereof is connected to the positive electrode terminal26. The negative electrode 20 is connected via a negative lead (notshown) to the cylindrical case 10 functioning as a negative terminal.

The positive electrode (cathode) active materials are well-known in theart. The positive electrode 16 can be manufactured by the steps of (a)mixing a positive electrode active material with a conductor agent(conductivity-promoting ingredient) and a binder, (b) dispersing theresultant mixture in a suitable solvent, (c) coating the resultingsuspension on a collector, and (d) removing the solvent from thesuspension to form a thin plate-like electrode. The positive electrodeactive material may be selected from a wide variety of oxides, such asmanganese dioxide, lithium/manganese composite oxide, lithium-containingnickel oxide, lithium-containing cobalt oxide, lithium-containing nickelcobalt oxide, lithium-containing iron oxide and lithium-containingvanadium oxide. Positive electrode active material may also be selectedfrom chalcogen compounds, such as titanium disulfate or molybdenumdisulfate. More preferred are lithium cobalt oxide (e.g., Li_(x)CoO₂where 0.8≦x≦1), lithium nickel oxide (e.g., LiNiO₂) and lithiummanganese oxide (e.g., LiMn₂O₄ and LiMnO₂) because these oxides providea high cell voltage.

Acetylene black, carbon black, or ultra-fine graphite particles may beused as a conductor agent. The binder may be chosen frompolytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF),ethylene-propylene-diene copolymer (EPDM), or styrene-butadiene rubber(SBR), for example. Conductive materials such as electronicallyconductive polymers, meso-phase pitch, coal tar pitch, and petroleumpitch may also be used. Preferable mixing ratio of these ingredients maybe 80 to 95% by weight for the positive electrode active material, 3 to20% by weight for the conductor agent, and 2 to 7% by weight for thebinder. The current collector may be selected from aluminum foil,stainless steel foil, and nickel foil. There is no particularlysignificant restriction on the type of current collector, provided thematerial is a good electrical conductor and relatively corrosionresistant. The separator may be selected from a synthetic resin nonwovenfabric, porous polyethylene film, porous polypropylene film, or porousPTFE film.

The negative electrode (anode), which the instant invention provides, isnow explained in detail as follows: Carbon materials can assume anessentially amorphous structure (glassy carbon), a highly organizedcrystal (graphite crystal or crystallite), or a whole range ofintermediate structures that are characterized by having variousproportions and sizes of graphite crystallites and defects dispersed inan amorphous carbon matrix. Typically, a graphite crystallite iscomposed of a number of graphene sheets or basal planes (also referredto as a-b planes) that are bonded together through van der Waals forcesin the c-axis direction, the direction perpendicular to the basal plane.These graphite crystallites are typically micron- or nanometer-sized inthe a- or b-direction (these are called La dimension). The c-directionaldimension (or thickness) is commonly referred to as Lc. The interplanarspacing of a perfect graphite is known to be approximately 0.335 nm(3.35 Å). The graphite crystallites are dispersed in or connected bycrystal defects or an amorphous phase in a graphite particle, which canbe a graphite flake, carbon/graphite fiber segment, carbon/graphitewhisker, or carbon/graphite nano-fiber. In the case of a carbon orgraphite fiber segment, the graphene plates may be a part of acharacteristic “turbostratic” structure.

According to a preferred embodiment of the present invention, a negativeelectrode (anode) material composition for use in a lithium secondarybattery may comprise a carbonaceous material that is capable ofabsorbing and desorbing lithium ions. This carbonaceous materialcomprises a graphite crystal structure having an interplanar spacingd₀₀₂ of at least 0.400 nm, which is derived from a measured (002)reflection peak in powder X-ray diffraction. This interplanar spacing,also referred to as inter-layer or inter-graphene spacing (C/2 in agraphite unit cell structure), is preferably at least 0.55 nm and mostpreferably greater than 0.6 nm. Such an expanded interstitial space issubstantially permanent as a result of a chemical treatment to agraphite crystal-containing material, such as natural graphite. This isas opposed to most of the intercalation treatments to form a graphiteintercalation compound (GIC), in which the interplanar placing restoresto approximately the original magnitude once the intercalant species inthe graphite intercalation compound are removed. Conventional GICs areprepared primarily for the purpose of producing flexible graphitethrough exfoliation of GICs.

For instance, a preferred embodiment of the present invention is acarbonaceous material composition, referred to as graphite fluoride (GF)with a chemical formula [CF_(x)]_(n), where 0.5≦x≦1.0. This chemicalsubstance comprises the lamellar structure of carbon atoms in a graphitelattice with atoms of fluorine interposed between the graphene planes.However, GF differs from fluorine-intercalated graphite in severalrespects. For one, the fluorine is not easily removed from the graphitefluoride by simple heating to temperatures at which the intercalatedproduct would freely release its intercalate. Typically, theintercalated product will freely release most of the fluorineintercalate at temperatures in the range of 350° C. to 400° C. Thecommonly produced graphite fluoride is thought to be a mixture of twodifferent compounds, [CF]_(n) and [C₂F]_(n) having interplanar spacingsof 5.8-5.9 Å and 8.8-9.0 Å, respectively. Depending upon the preparationconditions, the average interplanar spacing of a GF sample can varybetween these two values. The fluorine atoms in graphite fluoride arecovalently bonded to the carbon atoms, not just residing in theinterstitial spaces as is true in fluorine-intercalated graphite. Thefluorine-intercalated product has a higher conductivity than does thepristine graphite from which it is made. In contrast, graphite fluoridehas a conductivity that is lower than that of the pristine graphite fromwhich it is made.

In one preferred embodiment, the carbonaceous material comprises amaterial derived from natural graphite, synthetic graphite, highlyoriented pyrolytic graphite, graphite fiber, carbon fiber, carbonnano-fiber, graphitic nano-fiber, spherical graphite or graphiteglobule, meso-phase micro-bead, meso-phase pitch, graphitic coke, orpolymeric carbon. For instance, natural flake graphite may be subjectedto a deep oxidation treatment under a condition comparable to what hasbeen commonly employed to prepare the so-called expandable graphite orstable graphite intercalation compound, but with a higher degree ofoxidation. This can be accomplished by immersing graphite powder in asolution of sulfuric acid, nitric acid or nitrate, and potassiumpermanganate for preferably 1-24 hours (details to be described later).The resulting acid-intercalated graphite compound is then subjected tovigorous washing and rinsing to remove essentially all the intercalants.The subsequently dried product is a heavily oxidized graphite powder,which comprises graphite oxide. Powder X-ray diffraction indicates thatthe interplanar spacing is typically between 0.55 and 0.75 nm. Theseexpanded interplanar spacings provide interstitial spaces that are ideallocations to accommodate lithium ions or atoms. They appear to be largeenough to accommodate at least two “layers” of lithium, as opposed tojust one layer of lithium atoms in a conventional interstitial space(0.27 nm) with an interplanar spacing of typically 0.335 nm.

It may be noted that Tossici, et al, U.S. Pat. No. 6,087,043 (Jul. 11,2000) [Ref. 18], intercalated graphite with slightly larger potassiumatoms to form a first-stage graphite intercalation compound, KC₈ for useas a starting anode active material. It was believed by Tossici, et al.that, during the first discharge cycle, most of the potassium ions werereleased, leaving behind slightly expanded interstitial spaces, with aninterplanar spacing of 0.341 nm (an increase by only 0.06 nm). Theexpanded interplanar spacing appears to “enhance the kinetics of theelectrochemical process, thus resulting in a high-rate carbon anode”[Ref. 18]. However, pre-intercalation with potassium atoms did not leadto an increased specific capacity. In contrast, the presently inventedcarbonaceous materials with significantly expanded interstitial spacesprovide a dramatically enhanced specific capacity: typically 500-850mAh/g or higher as opposed to typically 225-350 mAh/g (albeit with atheoretical maximum of 372 mAh/g) for untreated graphite or potassiumpre-intercalated graphite.

The carbonaceous material may be derived from particles or flakes ofnatural graphite, synthetic graphite, highly oriented pyrolyticgraphite, graphite fiber, carbon fiber, carbon nano-fiber, graphiticnano-fiber, spheroidal graphite or graphite globule, meso-carbonmicro-bead (MCMB), meso-phase pitch, graphitic coke, or polymericcarbon. The spheroidal graphite, produced by spheroidizing naturalgraphite flakes using a special thermo-chemical procedure, is availablefrom several commercial sources (e.g., Huadong Graphite Co., Pingdu,China). The spheroidal graphite has a basically identical crystallinestructure as in natural graphite, having relatively well-orderedcrystallites with an interplanar spacing of 0.336 nm. The MCMB isobtained by extracting meso-phase particles out of other less-orderedcarbon matrix and then graphitizing the meso-phase particles. They aretypically supplied as a highly graphitic form of graphite. Commercialsources of MCMBs include Alumina Trading (the U.S. distributor for thesupplier, Osaka Gas Company of Japan) and Shanghai Shanshan Tech, China.Both the MCMB and spheroidal graphite may be subjected to the sameinterstitial space expanding treatment as natural graphite.

Although both non-graphitic carbon materials or graphitic carbonmaterials may be employed in practicing the present invention, graphiticmaterials, such as natural graphite, spheroidal natural graphite,meso-carbon microbeads, and carbon fibers (such as mesophase carbonfibers), are preferably used. The carbonaceous material preferably has anumerical particle size (measured by a laser scattering method) that issmaller than about 25 μm, more preferably smaller than about 15 μm,further preferably smaller than about 10 μm, and most preferably smallerthan about 6 μm. The smaller particle size reduces lithium diffusiondistances and increases rate capability of the anode, which is a factorin preventing lithium plating at the anode. In those instances where theparticle is not spherical, the length scale parallel to the direction oflithium diffusion is the figure of merit. Larger particle sizedmaterials may be used if the lithium diffusion coefficient is high. Thediffusion coefficient of MCMB is about. 10⁻¹⁰ cm²/s. Synthetic graphitehas a diffusion coefficient of about 10⁻⁸ cm²/s. Hence, larger particlesizes graphite could be used if synthetic graphite is chosen.

Meso-phase pitch, graphitic coke, or polymeric carbon may requireadditional graphitization treatment, typically at a temperature in therange of 1,500 to 3,000° C. to form nano- or micro-crystallitesdispersed in an amorphous carbon matrix. Such a blend or composite ofgraphitic phase (graphite crystallites) is then subjected to the sameinterstitial space expanding treatment; e.g., via a deep oxidation orfluorination procedure. The effects of this treatment include expandingthe interstitial spaces in the graphite crystallites. Further, wespeculate that, just like the mild oxidation treatment, the presentlyinvented deep oxidation or fluorination treatment can serve to (1)remove some active sites or defects in graphitic materials, resulting inan improved surface structure and (2) form a dense layer of oxides orfluorides acting as an efficient passivating film. Both effects willsignificantly reduce the magnitude of irreversible capacity loss afterfirst and subsequent cycles. Thus, preferably, the carbonaceous materialcomprises graphite oxide or graphite fluoride, or domains of graphiteoxide or graphite fluoride in an amorphous carbon matrix.

A graphite material (e.g., natural graphite particle) is typicallycomposed of graphite crystals dispersed in or connected to an amorphousphase, along with other defects. The proportion of the amorphous carbonphase (the disordered content) of a graphite material may be increasedin the following manner: Graphite particles (e.g., natural graphiteflakes, MCMB particles, or artificially made graphite globules) may bemixed with a resin to form a composite. This composite may be heated toa temperature of typically 500-1,000° C. for a sufficient period of timeto convert the resin into a polymeric carbon or an amorphous carbonphase. Hence, in the presently invented negative electrode materialcomposition, the carbonaceous material may further comprise an amorphouscarbon phase or polymeric carbon, wherein the graphite particle or agraphite crystal structure therein is dispersed in or bonded by anamorphous carbon phase or polymeric carbon.

Alternatively, the amorphous carbon phase may be obtained from chemicalvapor deposition (CVD), chemical vapor infiltration (CVI), orpyrolyzation of an organic precursor. CVD or CVI techniques arewell-known in the art and have been utilized to cover a graphitematerial with an amorphous coating [e.g., Refs. 19-21]. Pyrolyzation ofpolymer-bonded graphite particles was studied by Kawakubo, et al. [Ref.22]. However, these techniques have never been used to cover graphiteparticles or carbonaceous materials containing graphite crystalliteswith significantly expanded interstitial spaces. This is not a trivialtask or an obvious extension of conventional CVD, CVI, or pyrolyzationtreatments because a skilled person in the art would expect theunderlying carbonaceous materials with expanded interstitial spaces(e.g., graphite oxide or graphite fluoride) to undergo significantchemical changes when subjected to a CVD or pyrolyzation treatment at atemperature typically higher than 500° C. in each case. Such a skilledperson in the art would further expect that the resulting graphitematerial may return its interplanar spacing perhaps back to 0.335 nm.Surprisingly, this was not the case. After CVD or pyrolyzation treatmentat a temperature lower than 750° C., no significant change in theinterplanar spacing of expanded graphite materials studied was observed.

Further alternatively, the carbonaceous material may comprise anelectrically conductive binder material, wherein the graphite crystalstructure is dispersed in or bonded by this conductive binder material.An electrically conductive binder material may be selected from coal tarpitch, petroleum pitch, meso-phase pitch, coke, a pyrolyzed version ofpitch or coke, or a conjugate chain polymer (intrinsically conductivepolymer such as polythiophene, polypyrrole, or polyaniline.

In the preparation of a negative electrode material, typically carbonparticles are bonded by a non-conductive material, such aspolyvinylidene fluoride (PVDF), to form an integral anode member. Hence,the carbonaceous material may further comprise a non-conductive bindermaterial, wherein the graphite crystal structure is dispersed in orbonded by the non-conductive binder material.

A wide range of electrolytes can be used for practicing the instantinvention. Most preferred are non-aqueous and polymer gel electrolytesalthough other types can be used. The non-aqueous electrolyte to beemployed herein may be produced by dissolving an electrolytic salt in anon-aqueous solvent. Any known non-aqueous solvent which has beenemployed as a solvent for a lithium secondary battery can be employed. Anon-aqueous solvent mainly consisting of a mixed solvent comprisingethylene carbonate (EC) and at least one kind of non-aqueous solventwhose melting point is lower than that of aforementioned ethylenecarbonate and whose donor number is 18 or less (hereinafter referred toas a second solvent) may be preferably employed. This non-aqueoussolvent is advantageous in that it is (a) stable against a negativeelectrode containing a carbonaceous material well developed in graphitestructure; (b) effective in suppressing the reductive or oxidativedecomposition of electrolyte; and (c) high in conductivity. Anon-aqueous electrolyte solely composed of ethylene carbonate (EC) isadvantageous in that it is relatively stable against decompositionthrough a reduction by a graphitized carbonaceous material. However, themelting point of EC is relatively high, 39 to 40° C., and the viscositythereof is relatively high, so that the conductivity thereof is low,thus making EC alone unsuited for use as a secondary battery electrolyteto be operated at room temperature or lower. The second solvent to beused in a mixture with EC functions to make the viscosity of the solventmixture lower than that of EC alone, thereby promoting the ionconductivity of the mixed solvent. Furthermore, when the second solventhaving a donor number of 18 or less (the donor number of ethylenecarbonate is 16.4) is employed, the aforementioned ethylene carbonatecan be easily and selectively solvated with lithium ion, so that thereduction reaction of the second solvent with the carbonaceous materialwell developed in graphitization is assumed to be suppressed. Further,when the donor number of the second solvent is controlled to not morethan 18, the oxidative decomposition potential to the lithium electrodecan be easily increased to 4 V or more, so that it is possible tomanufacture a lithium secondary battery of high voltage.

Preferable second solvents are dimethyl carbonate (DMC), methylethylcarbonate (MEC), diethyl carbonate (DEC), ethyl propionate, methylpropionate, propylene carbonate (PC), gamma.-butyrolactone (.gamma.-BL),acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methylformate (MF), toluene, xylene and methyl acetate (MA). These secondsolvents may be employed singly or in a combination of two or more. Moredesirably, this second solvent should be selected from those having adonor number of 16.5 or less. The viscosity of this second solventshould preferably be 28 cps or less at 25° C.

The mixing ratio of the aforementioned ethylene carbonate in the mixedsolvent should preferably be 10 to 80% by volume. If the mixing ratio ofthe ethylene carbonate falls outside this range, the conductivity of thesolvent may be lowered or the solvent tends to be more easilydecomposed, thereby deteriorating the charge/discharge efficiency. Morepreferable mixing ratio of the ethylene carbonate is 20 to 75% byvolume. When the mixing ratio of ethylene carbonate in a non-aqueoussolvent is increased to 20% by volume or more, the solvating effect ofethylene carbonate to lithium ions will be facilitated and the solventdecomposition-inhibiting effect thereof can be improved.

Examples of preferred mixed solvent are a composition comprising EC andMEC; comprising EC, PC and MEC; comprising EC, MEC and DEC; comprisingEC, MEC and DMC; and comprising EC, MEC, PC and DEC; with the volumeratio of MEC being controlled within the range of 30 to 80%. Byselecting the volume ratio of MEC from the range of 30 to 80%, morepreferably 40 to 70%, the conductivity of the solvent can be improved.With the purpose of suppressing the decomposition reaction of thesolvent, an electrolyte having carbon dioxide dissolved therein may beemployed, thereby effectively improving both the capacity and cycle lifeof the battery.

The electrolytic salts to be incorporated into a non-aqueous electrolytemay be selected from a lithium salt such as lithium perchlorate(LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride(LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-metasulfonate (LiCF₃SO₃) and bis-trifluoromethyl sulfonylimidelithium [LiN(CF₃SO₂)₂]. Among them, LiPF₆, LiBF₄ and LiN(CF₃SO₂)₂ arepreferred. The content of aforementioned electrolytic salts in thenon-aqueous solvent is preferably from 0.5 to 2.0 mol/l.

Example 1

Natural flake graphite, nominally sized at 45 μm, provided by AsburyCarbons (405 Old Main St., Asbury, N.J. 08802, USA) was milled to reducethe size to approximately 14 μm (Sample 1a). The chemicals used in thepresent study, including fuming nitric acid (>90%), sulfuric acid(95-98%), potassium chlorate (98%), and hydrochloric acid (37%), werepurchased from Sigma-Aldrich and used as received. Graphite oxide (GO)samples were prepared according to the following procedure:

Sample 1A: A reaction flask containing a magnetic stir bar was chargedwith sulfuric acid (176 mL) and nitric acid (90 mL) and cooled byimmersion in an ice bath. The acid mixture was stirred and allowed tocool for 15 min, and graphite (10 g) was added under vigorous stirringto avoid agglomeration. After the graphite powder was well dispersed,potassium chlorate (110 g) was added slowly over 15 min to avoid suddenincreases in temperature. The reaction flask was loosely capped to allowevolution of gas from the reaction mixture, which was stirred for 24hours at room temperature. On completion of the reaction, the mixturewas poured into 8 L of deionized water and filtered. The GO wasre-dispersed and washed in a 5% solution of HCl to remove sulphate ions.The filtrate was tested intermittently with barium chloride to determineif sulphate ions are present. The HCl washing step was repeated untilthis test was negative. The GO was then washed repeatedly with deionizedwater until the pH of the filtrate was neutral. The GO slurry wasspray-dried and stored in a vacuum oven at 60° C. until use.

Sample 1B: The same procedure as in Sample 1A was followed, but thereaction time was 48 hours.

Sample 1C: The same procedure as in Sample 1A was followed, but thereaction time was 96 hours.

X-ray diffraction studies (FIG. 2) showed that after a treatment of 24hours, a significant proportion of graphite has been transformed intographite oxide. The peak at 2θ=26.3°, corresponding to an interplanarspacing of 0.335 nm (3.35 Å) for pristine natural graphite (top curve inFIG. 2), was significantly reduced in intensity (middle curve) after adeep oxidation treatment for 24 hours. The curves for treatment times of48 and 96 hours are essentially identical (bottom curve), showing thatessentially all of the graphite crystals have been converted intographite oxide with an interplanar spacing of 6.5-7.5 Å (the 26.3° peakhas totally disappeared).

Example 2

Samples 2A, 2B, 2C, and 2D were prepared according to the same procedureused for Sample 1B, but the starting graphite materials were highlyoriented pyrolytic graphite (HOPG), graphite fiber, graphitic carbonnano-fiber, and spheroidal graphite, respectively. Their finalinterplanar spacings are 6.6 Å, 7.3 Å, 7.3 Å, and 6.6 Å, respectively.Their un-treated counterparts are referred to as Sample 2a, 2b, 2c, and2d, respectively.

Example 3

Graphite oxide (Sample 3A) was prepared by oxidation of natural graphiteflakes (original size of 200 mesh, from Huadong Graphite Co., Pingdu,China, milled to approximately 15 μm, referred to as Sample 3a) withsulfuric acid, sodium nitrate, and potassium permanganate according tothe method of Hummers [U.S. Pat. No. 2,798,878, Jul. 9, 1957]. In thisexample, for every 1 gram of graphite, we used a mixture of 22 ml ofconcentrated sulfuric acid, 2.8 grams of potassium permanganate, and 0.5grams of sodium nitrate. The graphite flakes were immersed in themixture solution and the reaction time was approximately one hour at 35°C. It is important to caution that potassium permanganate should begradually added to sulfuric acid in a well-controlled manner to avoidoverheat and other safety issues. Upon completion of the reaction, themixture was poured into deionized water and filtered. The sample wasthen washed repeatedly with deionized water until the pH of the filtratewas approximately 5. The slurry was spray-dried and stored in a vacuumoven at 60° C. for 24 hours. The interlayer spacing of the resultinglaminar graphite oxide was determined by the Debye-Scherrer X-raytechnique to be approximately 0.73 nm (7.3 Å).

Example 4

Graphite oxide (Sample 4A) was prepared by oxidation of meso-carbonmicro-beads (MCMBs) according to the same procedure used in Example 3.MCMB 2528 microbeads (Sample 4a) were supplied by Alumina Trading, whichis the U.S. distributor for the supplier, Osaka Gas Company of Japan.This material has a density of about 2.24 g/cm³; a particle size maximumfor at least 95% by weight of the particles of 37 microns; median sizeof about 22.5 microns and an interplanar distance of about 0.336 nm.After deep oxidation treatment, the interplanar spacing in the resultinggraphite oxide micro-beads is approximately 0.76 nm.

Example 5

Amoco P-100 graphitized carbon fiber (Sample 5a), having an interplanarspacing of 3.37 Å (0.337 nm) and a fiber diameter of 10 μm was firsthalogenated with a combination of bromine and iodine at temperaturesranging from 75° C. to 115° C. to form a bromine-iodine intercalationcompound of graphite as an intermediate product. The intermediateproduct was then reacted with fluorine gas at temperatures ranging from275° C. to 450° C. to form the CF_(x). The value of x in the CF_(x)samples was approximately 0.6-0.9. X-ray diffraction curves typicallyshow the co-existence of two peaks corresponding to 0.59 nm and 0.88 nm,respectively. Sample 5A exhibits substantially 0.59 nm peak only andSample 5B exhibits substantially 0.88 nm peak only.

Example 6

A CF_(0.68) sample obtained in EXAMPLE 5 was exposed at 250° C. and 1atmosphere to vapors of 1,4-dibromo-2-butene (BrH₂C—CH═CH—CH₂Br) for 3hours. It was found that two-thirds of the fluorine was lost from thegraphite fluoride sample. It is speculated that 1,4-dibromo-2-buteneactively reacts with graphite fluoride, removing fluorine from thegraphite fluoride and forming bonds to carbon atoms in the graphitelattice. The resulting product (Sample 6A) is mixed halogenatedgraphite, likely a combination of graphite fluoride and graphitebromide.

Example 7

Natural graphite flakes, a sieve size of 200 to 250 mesh, were heated invacuum (under less than 10⁻² mmHg) for about 2 hours to remove theresidual moisture contained in the graphite. Fluorine gas was introducedinto a reactor and the reaction was allowed to proceed at 375° C. for120 hours while maintaining the fluorine pressure at 200 mmHg. This wasbased on the procedure suggested by Watanabe, et al. disclosed in U.S.Pat. No. 4,139,474) [Ref. 25]. The powder product obtained was black incolor. The fluorine content of the product was measured as follows. Theproduct was burnt according to the oxygen flask combustion method andthe fluorine was absorbed into water as hydrogen fluoride. The amount offluorine was determined by employing a fluorine ion electrode. From theresult, we obtained a GF (Sample 7A) having an empirical formula(CF_(0.58))_(n). X-ray diffraction indicated a major (002) peak at2θ=10°, corresponding to an interplanar spacing of 9.0 Å.

Sample 7B was obtained in a manner similar to that for Sample 7A, but ata reaction temperature of 640 C for 5 hours. The chemical compositionwas determined to be (CF_(0.93))_(n). X-ray diffraction indicated amajor (002) peak at 2θ=13.5°, corresponding to an interplanar spacing of5.85 Å.

Example 8

Two polymeric carbon-coated GO samples (Samples 8-A and 8-B) wereprepared by mixing GO particles (prepared in Example 3 and Example 4,respectively) with a phenol resin to obtain a mixture comprising 20% byvolume of phenol resin in each case. The mixture was cured at 200° C.for one hour and then carbonized in an argon atmosphere at a temperatureof 500° C. Then, the carbonized product was ground and milled to obtainparticles of 1 to 23 μm, with an average diameter of approximately 13μm. Surprisingly, the interplanar spacing was determined to remainapproximately the same (0.73 nm and 0.76 nm, respectively) even thoughthe GO particles have experienced a temperature as high as 500° C.

Example 9

Natural flake graphite, milled to an average size of approximately 14microns, was subjected to the same fluorination treatment as describedin Example 7 and determined to be CF_(0.58) (Sample 7A). The resultingpowder was subjected to a chemical vapor deposition (CVD) of amorphouscarbon according to a procedure suggested by Tanaka, et al., U.S. Pat.No. 5,344,726 [Ref. 19]. A CF_(0.58) sample powder of 50 mg was placedin a quartz tube reactor, and then argon gas and propane gas weresupplied from an argon supply line and a propane supply line,respectively. Then, a propane concentration of raw gas was set to 10mole % by handling needle valves. Flow velocity of the raw gas was setto 12.7 cm/min and an amount of supply of propane was set to 0.05 mol/h.It may be noted that a hydrocarbon or its derivatives other than propanemay be used as a raw material. More specifically, an aliphatichydrocarbon, an aromatic hydrocarbon, an alicyclic hydrocarbon or thelike may be used. Further specifically, methane, ethane, butane,benzene, toluene, naphthalene, acetylene, biphenyl and substitutionproducts thereof may be used. The powder was heated by a furnace atapproximately 750° C., whereby propane supplied from a pyrex tube waspyrolytically decomposed with a pyrolysis carbon being deposited on thesurface of the graphite fluoride powder. The resulting material wasmilled to become fine particles of approximately 16.5 microns, which areessentially amorphous carbon-coated GF particles (Sample 9A).

Example 10

Sample 10A was identical to Sample 3A, which was subsequently bonded bya conducting polymer-based binder resin to prepare a sample for Example20.

EXAMPLES 11-20

The anode active materials prepared in Examples 1-10 were separatelyincorporated into a lithium ion battery to prepare samples for Examples11-20, respectively. The cathode of a lithium ion battery was preparedin the following way. First of all, 91% by weight of lithium cobaltoxide powder LiCoO₂, 3.5% by weight of acetylene black, 3.5% by weightof graphite, and 2% by weight of ethylene-propylene-diene monomer powderwere mixed together with toluene to obtain a mixture. The mixture wasthen coated on an aluminum foil (30 μm) serving as a current collector.The resulting two-layer aluminum foil-active material configuration wasthen hot-pressed to obtain a positive electrode.

The powder particles obtained in Examples 1-9 were separately mixedwith, as a binder, 2.2% by weight of styrene/butadiene rubber and 1.1%by weight of carboxylmethyl cellulose to obtain a mixture (a precursorto an anode active material), which was then coated on a copper foil tobe employed as a collector. After being dried, the powder/resinmixture-copper foil configuration was hot-pressed to obtain a negativeelectrode (in Examples 11-19).

In Example 20, a portion of the powder particles prepared for Sample 1Dwas mixed with an electronically conductive polymer, polyaniline.Polyaniline-maleic acid-dodecylhydrogensulfate salt was synthesizeddirectly via emulsion polymerization pathway using benzoyl peroxideoxidant, sodium dodecyl sulfate surfactant, and maleic acid as dopants.Dry polyaniline powder was dissolved in DMF up to 2% w/v to form asolution. The graphite oxide particles were then dispersed in thissolution to form a suspension, which was then coated on a copper foil tobe employed as a collector. Upon removal of the solvent, thepowder/resin mixture-copper foil configuration was hot-pressed to obtaina negative electrode.

A positive electrode, a separator composed of a porous polyethylenefilm, and a negative electrode was stacked in this order. The stackedbody was spirally wound with a separator layer being disposed at theoutermost side to obtain an electrode assembly as schematically shown inFIG. 1.

Hexafluorolithium phosphate (LiPF₆) was dissolved in a mixed solventconsisting of ethylene carbonate (EC) and methylethyl carbonate (MEC)(volume ratio: 50:50) to obtain a non-aqueous electrolyte, theconcentration of LiPF₆ being 1.0 mol/l (solvent).

Finally, the electrode assembly and the non-aqueous electrolyte wereplaced in a bottomed cylindrical case made of stainless steel, therebyobtaining a cylindrical lithium secondary battery as shown in FIG. 1.

In order to compare the electrochemical behaviors of carbonaceous anodematerials prepared in Examples 1-10, we used a method analogous to thatused by Wu, et al. [Ref. 12]. The results are summarized in Table 1.

TABLE 1 Specific capacity and reversible specific capacity ofcarbonaceous anode active materials. Specific Reversible Reten- SampleAnode active Interplanar capacity capacity tion No. Material spacings, ÅmAh/g mAh/g ratio, % 1a Natural graphite 3.35 260 184 70.8 1A GO, 24 hrs3.35--7.5 545 495 90.8 1B GO, 48 hrs  6.5--7.5 742 652 87.9 1C GO, 96hrs  6.5--7.5 752 662 88.0 2a HOPG 3.35 285 205 71.9 2A HOPG oxide 6.6685 623 90.9 2b Graphite fiber 3.4 343 285 83.1 2B Oxidized GF 7.3 764654 85.6 2c CNF 3.36 264 189 71.6 2C Oxidized CNF 7.3 712 630 88.5 2dSpheroidal Gr 3.35 285 202 70.9 2D Oxidized S-Gr 7.5 747 665 89.0 3aNatural graphite 3.35 265 190 71.7 3A GO, Hummers 7.3 732 644 88.0 4aMCMB 3.36 310 270 87.1 4A Oxidized MCMB 7.6 756 667 88.2 5a Graphitefiber 3.4 325 280 86.2 5A CF_(0.9) 5.9 635 576 90.7 5B CF_(0.6) 8.8 840760 90.5 6A CBrF_(x) 8.4 792 721 91.0 7A CF_(0.58) 9 856 771 90.1 7BCF_(0.93) 5.85 643 575 89.4 8A 3A + C-coated 7.3 762 657 86.2 8B 4A +C-coated 7.6 782 685 87.6 9A 7A + CVD-C 9 875 785 89.7 10A PANi-bonded7.3 732 645 88.1

The following significant observations are made from Table 1 and relatedcharts (FIG. 3-FIG. 5):

-   -   (1) In every group of carbonaceous anode materials studied, both        the specific capacity and reversible capacity (after first        cycle) of materials with expanded interstitial spaces        (interplanar spacing greater 0.55 nm or 5.5 Å) are significantly        higher than those of their un-expanded counterparts. For        instance, 1A, 1B, 1C, and 1D (graphite oxide) are all greater        than 1a (natural graphite).    -   (2) The retention ratio, defined as the reversible        capacity/specific capacity, typically 85% to 91% for a treated        sample (with expanded interstitial spaces) is always better than        that (typically 70% to 83%) for a corresponding un-treated        sample.    -   (3) Both the total specific capacity (FIG. 3) and reversible        capacity (FIG. 4) appear to increase with increasing interplanar        spacing when all the data points for Samples 1-10 are plotted on        the same chart (regardless the type of carbonaceous materials).        If we extrapolate the curve in FIG. 3 and draw a horizontal        straight line starting at a vertical axis coordinate point of        372 mAh/g, this straight line and the data curve intersect at an        interplanar spacing of approximately 4.0 Å or 0.4 nm. This        implies that any graphitic material with an expanded        interstitial space (such that the interplanar spacing is 4.0 Å        or greater) will likely exhibit a specific capacity greater than        the theoretical capacity of its un-expanded counterpart. Hence,        it is fair to say that the present invention provides a powerful        platform technology for enhancing the specific capacity of        carbonaceous anode materials.    -   (4) The data for Samples 8A, 8B, and 9A demonstrate that an        amorphous carbon coating on graphitic carbon particles can        further enhance the electrochemical responses of the materials        with expanded interstitial spaces. This coating may be obtained        via carbonization of a polymer or chemical vapor deposition of        carbon.    -   (5) As demonstrated in FIG. 5, the treated carbonaceous material        with expanded interstitial spaces is also more capable of        retaining the specific capacity as the secondary battery        undergoes cycles of charge and discharge. It is speculated that        the associated treatment has provided a stable solid electrolyte        interface (SEI) that has effectively prevented the continued        consumption of lithium ions in forming irreversible lithium        compounds. Those who are skilled in the art would have predicted        that the expanded interstitial spaces favor the co-intercalation        of solvents, which has been one of the primary causes of        capacity fade. To the contrary and quite surprisingly, the        expanded interstitial spaces (with an interplanar spacing up to        9.0 Å) did not seem to have any solvent co-intercalation issue.

1. A lithium secondary battery comprising a positive electrode, anegative electrode comprising a carbonaceous material which is capableof absorbing and desorbing lithium ions, and a non-aqueous electrolytedisposed between said negative electrode and said positive electrode;wherein said carbonaceous material comprises a graphite crystalstructure having an interplanar spacing d₀₀₂ of at least 0.400 nmderived from a (002) reflection peak in powder X-ray diffraction.
 2. Thelithium secondary battery as defined in claim 1, wherein saidinterplanar spacing is at least 0.55 nm.
 3. The lithium secondarybattery as defined in claim 1, wherein said carbonaceous materialcomprises a material derived from natural graphite, synthetic graphite,highly oriented pyrolytic graphite, graphite fiber, carbon fiber, carbonnano-fiber, graphitic nano-fiber, spherical graphite or graphiteglobule, meso-phase micro-bead, meso-phase pitch, graphitic coke, orpolymeric carbon.
 4. The lithium secondary battery as defined in claim2, wherein said carbonaceous material comprises a material derived fromnatural graphite, synthetic graphite, highly oriented pyrolyticgraphite, graphite fiber, carbon fiber, carbon nano-fiber, graphiticnano-fiber, spherical graphite or graphite globule, meso-phasemicro-bead, meso-phase pitch, graphitic coke, or polymeric carbon. 5.The lithium secondary battery as defined in claim 1, wherein saidcarbonaceous material comprises graphite oxide or graphite fluoride. 6.The lithium secondary battery according to claim 1, wherein saidpositive electrode comprises lithium cobalt oxide, lithium nickel oxide,lithium manganese oxide, or a combination thereof.
 7. The lithiumsecondary battery as defined in claim 1, wherein said carbonaceousmaterial further comprises an amorphous carbon phase or polymeric carbonwherein said graphite crystal structure is dispersed in or bonded bysaid amorphous carbon phase or polymeric carbon.
 8. The lithiumsecondary battery as defined in claim 7, wherein said amorphous carbonphase is obtained from chemical vapor deposition, chemical vaporinfiltration, or pyrolyzation of an organic precursor.
 9. The lithiumsecondary battery as defined in claim 1, wherein said carbonaceousmaterial further comprises an electrically conductive binder materialwherein said graphite crystal structure is dispersed in or bonded bysaid conductive binder material.
 10. The lithium secondary battery asdefined in claim 1, wherein said electrically conductive binder materialcomprises coal tar pitch, petroleum pitch, meso-phase pitch, coke, aconjugate chain polymer, or a derivative thereof.
 11. The lithiumsecondary battery as defined in claim 1, wherein said carbonaceousmaterial further comprises a non-conductive binder material and whereinsaid graphite crystal structure is dispersed in or bonded by saidnon-conductive binder material.
 12. The lithium secondary battery asdefined in claim 2, wherein said carbonaceous material provides aspecific capacity of no less than 500 mAh/g.
 13. The lithium secondarybattery as defined in claim 2, wherein said carbonaceous materialprovides a specific capacity of no less than 650 mAh/g.
 14. The lithiumsecondary battery as defined in claim 2, wherein said carbonaceousmaterial provides a specific capacity of no less than 750 mAh/g.
 15. Anegative electrode material composition for use in a lithium secondarybattery, wherein said composition comprises a carbonaceous materialwhich is capable of absorbing and desorbing lithium ions and saidcarbonaceous material comprises a graphite crystal structure having aninterplanar spacing d₀₀₂ of at least 0.400 nm derived from a (002)reflection peak in powder X-ray diffraction.
 16. The negative electrodematerial composition as defined in claim 15, wherein said interplanarspacing is at least 0.55 nm.
 17. The negative electrode materialcomposition as defined in claim 15, wherein said carbonaceous materialcomprises a material derived from natural graphite, synthetic graphite,highly oriented pyrolytic graphite, graphite fiber, carbon fiber, carbonnano-fiber, graphitic nano-fiber, spherical graphite or graphiteglobule, meso-phase micro-bead, meso-phase pitch, graphitic coke, orpolymeric carbon.
 18. The negative electrode material composition asdefined in claim 16, wherein said carbonaceous material comprises amaterial derived from natural graphite, synthetic graphite, highlyoriented pyrolytic graphite, graphite fiber, carbon fiber, carbonnano-fiber, graphitic nano-fiber, spherical graphite or graphiteglobule, meso-phase micro-bead, meso-phase pitch, graphitic coke, orpolymeric carbon.
 19. The negative electrode material composition asdefined in claim 15, wherein said carbonaceous material comprisesgraphite oxide or graphite fluoride.
 20. The negative electrode materialcomposition as defined in claim 15, wherein said carbonaceous materialfurther comprises an amorphous carbon phase or polymeric carbon, whereinsaid graphite crystal structure is dispersed in or bonded by saidamorphous carbon phase or polymeric carbon.
 21. The negative electrodematerial composition as defined in claim 20, wherein said amorphouscarbon phase is obtained from chemical vapor deposition, chemical vaporinfiltration, or pyrolyzation of an organic precursor.
 22. The negativeelectrode material composition as defined in claim 15, wherein saidcarbonaceous material further comprises an electrically conductivebinder material, wherein said graphite crystal structure is dispersed inor bonded by said conductive binder material.
 23. The negative electrodematerial composition as defined in claim 15, wherein said electricallyconductive binder material comprises coal tar pitch, petroleum pitch,meso-phase pitch, coke, a conjugate chain polymer, or a derivativethereof.
 24. The negative electrode material composition as defined inclaim 15, wherein said carbonaceous material further comprises anon-conductive binder material, wherein said graphite crystal structureis dispersed in or bonded by said non-conductive binder material.